Check if NEON extension are actually available

User may pass own compiler flags to configure which override those
provided by project through automake. Therefore it is possible for user
on ARM platform to pass CXXFLAGS=-mfpu=vfp which will effectively
disable NEON even though used compiler supports -mfpu=neon (since user
supplied flags take precedence compiler invocation will use flags:
-mfpu=neon -mfpu=vfp). Instead of checking whether compiler supports
-mfpu=neon flag, check if NEON extensions are available by checking if
__ARM_NEON is defined when compiling with -mfpu=neon and user supplied
flags combined.

Signed-off-by: Jan Palus <jpalus@fastmail.com>

configure.ac

@@ -178,10 +178,20 @@ case "${host_cpu}" in
     ;;
 
   arm*)
-
-    AX_CHECK_COMPILE_FLAG([-mfpu=neon], [neon=true], [neon=false], [$WERROR])
-    AM_CONDITIONAL([HAVE_NEON], $neon)
-    if $neon; then
+    SAVE_CXXFLAGS="$CXXFLAGS"
+    CXXFLAGS="-mfpu=neon $CXXFLAGS"
+    AC_MSG_CHECKING([for NEON support])
+    AC_COMPILE_IFELSE(
+    [AC_LANG_PROGRAM([], [[
+    #ifndef __ARM_NEON
+           #error
+    #endif
+    ]])],
+    [neon=yes], [neon=no])
+    AC_MSG_RESULT([$neon])
+    CXXFLAGS="$SAVE_CXXFLAGS"
+    AM_CONDITIONAL([HAVE_NEON], test "xyes" = "x$neon")
+    if test "xyes" = "$neon"; then
       AC_DEFINE([HAVE_NEON], [1], [Enable NEON instructions])
       NEON_CXXFLAGS="-mfpu=neon"
       AC_SUBST([NEON_CXXFLAGS])